4 Wherefore, beloved, you need not expect to hear from us what the Lord then refrained from telling His disciples, because they were still unable to bear them: but rather seek to grow in the love that is shed abroad in your hearts by the Holy Spirit who is given unto you; that, fervent in spirit, and loving spiritual things, you may be able, not by any sign apparent to your bodily eyes, or any sound striking on your bodily ears, but by the inward eyesight and hearing, to become acquainted with that spiritual light and that spiritual word which carnal men are unable to bear.
For that cannot be loved which is altogether unknown. But when what is known, in however small a measure, is also loved, by the self-same love one is led on to a better and fuller knowledge. If, then, you grow in the love which the Holy Spirit spreads abroad in your hearts, “He will teach you all truth;” or, as other codices have it, “He will guide you in all truth:” as it is said, “Lead me in Your way, O Lord, and I will walk in Your truth.” So shall the result be, that not from outward teachers will you learn those things which the Lord at that time declined to utter, but be all taught of God; so that the very things which you have learned and believed by means of lessons and sermons supplied from without regarding the nature of God, as incorporeal, and unconfined by limits, and yet not rolled out as a mass of matter through infinite space, but everywhere whole and perfect and infinite, without the gleaming of colors, without the tracing of bodily outlines, without any markings of letters or succession of syllables—your minds themselves may have the power to perceive.
Well, now, I have just said something which is perhaps of that same character, and yet you have received it; and you have not only been able to bear it, but have also listened to it with pleasure. But were that inward Teacher, who, while still speaking in an external way to the disciples, said, “I have still many things to say unto you, but you cannot bear them now,” wishing to speak inwardly to us of what I have said of the incorporeal nature of God in the same way as He speaks to the angels, who always behold the face of the Father, we should still be unable to bear them.
Accordingly, when He says, “He will teach you all truth,” or “will guide you into all truth,” I do not think the fulfillment is possible in any one's mind in this present life (for who is there, while living in this corruptible and soul-oppressing body, that can know all truth, when even the apostle says, “We know in part”?), but because it is effected by the Holy Spirit, of whom we have now received the earnest, that we shall attain also to the actual fullness of knowledge: whereof it is said by the same apostle, “But then face to face;” and, “Now I know in part, but then shall I know even as also I am known;” not as a thing which he knows fully in this life, but which, as a thing that would still be future on to the attainment of that perfection, the Lord promised us through the love of the Spirit, when He said, “He will teach you all truth,” or “will guide you unto all truth.”
Source: Tractates on the Gospel of John (New Advent)